Low-pressure processing operations including device fabrication processes take place in vacuum (low-pressure) chambers that include chucks for supporting substrates in near vacuum or other low-pressure environments. Typical operating pressures for low-pressure fabrication processes such as physical-vapor deposition (PVD) and chemical-vapor deposition (CVD) range from less than 0.1 mTorr to more than 10.0 Torr. Some substrate chucks merely provide a substrate support platform and rely on substrate weight to hold the substrates in place. Most chucks, however, actively secure the substrates in process positions with either mechanical or electrostatic clamps.
Some chucks are also involved with the processing of the substrates by producing electrical or magnetic fields and/or by regulating heat transfers to or from the substrates. Electrical fields (e.g., produced through radio-frequency or "RF" bias) can be used to generate or enhance a plasma as well as to direct plasma ions and control the energy of ions impinging on the substrate. Magnetic fields can also be used to influence the plasma or to magnetically orient magnetic films during plasma-assisted depositions or thermal annealing. Heat transfers can be used either to remove excess heat from the substrates produced during processing operations or to provide a controlled amount of substrate heating for assisting other processing operations. Some processing operations are best performed at fixed substrate temperatures or at substrate temperatures that are adjusted throughout different stages of the operations. During operations like thermal annealing and thermal chemical vapor deposition (CVD) processes, elevated substrate temperatures activate or actually accomplish the substrate processing.
Thermal deposition and thermally activated processes such as chemical-vapor deposition (CVD), metal-organic chemical-vapor deposition (MOCVD), and thermal annealing processes also require active substrate heating (e.g., up to 350.degree. C.). Even higher substrate temperatures (e.g., up to 450.degree. C.) may be required for physical-vapor deposition (PVD) reflow depositions of interconnect materials (e.g., Al or Cu) for void-free filling of high-aspect-ratio structures. While some plasma sputtering operations require active heating of substrates, other plasma sputtering operations may require active cooling of the substrates. Some thermal deposition processes such as MOCVD of high-dielectric constant BST materials may require chucks for active heating of substrates to temperatures as high as 650.degree. C. On the other hand, some fabrication processes such as some plasma etch processes require active substrate cooling to temperatures as low as -40.degree. C.
However, controlling substrate temperatures in near vacuum or other low-pressure environments (e.g., process pressures below 1.0 Torr) is quite difficult because heat does not transfer well between objects in such environments. For example, the conduction of heat between contiguous surfaces of a chuck body and the substrate in a low-pressure environment is slow and inefficient (resulting in large temperature offsets) because actual contact on an atomic scale between their surfaces is limited to a small fraction of their common areas, and gaps that separate the remaining common areas of their surfaces are sufficient to prevent effective heat transfer by thermal conduction.
Heating and cooling of substrates through radiational heat transfers are possible in low-pressure environments, particularly at elevated substrate and/or chuck temperatures; but radiational heat transfers are generally too slow at lower temperatures to maintain substrates at desired processing temperatures. Below 500.degree. C., which includes most chuck-based fabrication processes, radiational heat transfers are generally too inefficient to regulate substrate processing temperatures effectively and quickly.
Faster transfers are possible by introducing a gas, preferably an inert gas such as helium or argon or another suitable gas such as nitrogen or hydrogen, between the chuck body and the substrate. Although still at much less than atmospheric pressure (e.g., 1 Torr to 20 Torr), the gas (referred to as a "heat-transfer" or "backside" gas) sufficiently fills the small gaps and voids between the chuck body and the substrate to support significant heat transfer by thermal conduction between them. A seal formed between the mounting surface of the chuck body and a back surface of the substrate resists leakage of the gas into the rest of the processing chamber, which could disturb substrate processing operations.
U.S. Pat. No. 4,680,061 to Lamont, Jr. and U.S. Pat. No. 4,949,783 to Lakios et al. disclose examples of chucks that promote such heat transfers between chuck bodies and substrates using a heat-transfer gas. Lamont, Jr. traps the gas in a shallow cavity between a chuck body and a substrate using a raised rim seal that projects from a mounting surface of the chuck body into engagement with a back surface of the substrate. Lakios et al. disclose a similar sealing structure but provide for circulating the gas through the cavity for removing excess heat from the substrate by both thermal conduction and forced thermal convection.
Although the raised rim seals of Lamont, Jr. and Lakios et al. circumscribe large interior portions of their substrates' back surfaces, the remaining portions of the back surfaces, which are engaged by their raised rim seals or which lie beyond the seals, are not exposed to the heat-transfer gas in the same manner as the more interior portions of the back surfaces. This can result in temperature gradients approaching their substrates' peripheries and in processing nonuniformities of corresponding peripheral regions on their substrates' front surfaces. Also, mechanical clamps of Lamont, Jr. and Lakios et al. engage the peripheral portions of their substrates' front surfaces, effectively blocking the engaged portions from effective processing due to an exclusion zone.
Accordingly, the usual practice has been to define a peripheral exclusion zone on the front surfaces of substrates, which must subsequently be discarded as unusable for device fabrication. Considering the high cost of substrate manufacture, considerable savings can be realized by reducing or eliminating the exclusion zone and fabricating active devices over the entire front surfaces of substrates.